add a duration parameter to the blackframe filter

Description

I really like the features available in the new silencedetect filter and would like to see similar functions for video in the blackframe filter. The blackframe and the silencedetect appear to have comparable objectives but their support for parameters differ. I'd like to see a duration parameter (similar to silentdetect's duration parameter) added to the blackframe filter to help identify durations of black in addition to frames of black.